The Auxiliary Power Unit (APU) provides auxiliary power (pneumatic and electric) to the various systems in an aircraft and Main Engine Start. It is conventionally suspended by several rigid linkages that attach the APU to the fuselage generally in the tailcone structure.
The APU suspension system ensures that all the rigid body degrees of freedom (DOF) of the system are removed whilst allowing relative motion to absorb the thermal expansion of the APU and manufacturing and assembly tolerances. Additionally, it has the purpose of isolating the aircraft from the machine vibrations and vice-versa, for instance, isolating the APU from aerodynamic and inertia dynamic loads coming from the aircraft by creating a spring damper system that avoids dynamic coupling between the APU and the aircraft and minimises load amplifications at critical frequencies.
Known suspension systems comprise seven struts or suspension rods for attaching the APU to the tailcone structure. They also comprise three vibration isolators (VIs) attached to the end of the struts adjacent to the engine or auxiliary power unit for reducing the transmission of vibration and shocks from the APU to the aircraft structure and vice-versa. The vibration isolators comprise a housing in connection with the struts for enclosing an elastomeric element for absorbing said vibrations.
Furthermore it comprises three APU attachment brackets for joining the Vibration Isolators (VIs) to the APU. The struts are joined to an extension of the housing. Each vibration isolator and its corresponding APU attachment bracket is called a mount.
Finally, seven structure attachment brackets are located at the end of the struts adjacent to the fuselage tailcone structure for joining the struts to said fuselage structure.
The struts have, in some cases, double pin rod-ends allowing the transmission of moments into the struts. The disavantage of said attachment configuration is not only that moments are carried by the struts but also that the dynamic behaviour predictability is poor, leading to a high number of unexpected failures.
The entire suspension system is designed to withstand all vibration, shocks, inertia loads within the defined operational limits. Known suspension systems are designed for being fail-safe with regards to strut failures and hence with respect to the failure of any one of the seven suspension rods or their respective attachment brackets with the fuselage structure or with the vibration isolator housing and in case of failure of one strut, the rest of the struts, APU attachment brackets, vibration isolators and structure attachment brackets are designed to withstand limit loads.
In addition each strut is also sized with safe-life criteria that is to say, they are designed to withstand the whole life of the aircraft. The metallic components of the three APU mounts and the corresponding APU attachment bracket are also designed safe-life.
All other APU system interfaces are flexible to permit translational movement and rotational displacement of the APU in all directions.
In case of fire in the APU compartment, the design of the vibration isolator's core member and housing does not allow separation of these parts even if the elastomer component forming the vibration isolator is damaged or destroyed. Although the function of the vibrator isolators will then be lost, the APU will settle only slightly.
The two main limitations of this design approach are, on one hand, that in case any mount fails, the system as a whole fails, or in other words, the system is not fail-safe with respect to mount failures, potentially leading to an inadvertent APU detachment. On the other hand, the system also has poor reliability with lower than expected Mean Time Between Unscheduled Removals (MTBURs) owing to a high sensitivity to changes in the dynamic behaviour, related to the coupling with the tailcone or local resonances, and internal loads distribution. Moreover, none of the mounts or vibration isolators have fail-safe devices such that they would accommodate a failure of any of their sub-components.